1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an electrostatic discharge protection circuit for a liquid crystal display device.
2. Description of the Related Art
The display device has become very important as a visual information communicating media in information society. The Cathode Ray Tube (CRT), which has been the mainstream display device, typically has a problematic large weight and bulk. Accordingly, various flat panel display devices have been developed capable of eliminating the weight and bulk disadvantages associated with CRT based displays.
Flat panel display devices include a liquid crystal display device (hereinafter, referred to as “LCD”), a field emission display (hereinafter, referred to as “FED”), a plasma display panel (hereinafter, referred to as “PDP”), and an electro-luminescence (hereinafter, referred to as “EL”). Most of these flat panel display devices have been put to practical use and have a significant share of the display device market.
Because the liquid crystal display devices can be produced with thin profiles and light weight electronics and because of process improvements in the manufacturing the liquid crystal display devices, the cathode ray tube has rapidly been replaced with the liquid crystal display device in many application areas.
Liquid crystal display devices control light transmittance of liquid crystal cells in accordance with a video signal to display a picture corresponding to the video signal on a liquid crystal display panel having liquid crystal cells arranged in a matrix. To this end, a liquid crystal display device includes a liquid crystal display panel having liquid crystal cells arranged in a matrix, and driving circuits that drive the liquid crystal display panel. The liquid crystal cells are defined by the crossing of gate lines with data lines.
The liquid crystal display panel includes a thin film transistor array substrate and a color filter array substrate that are bonded together with having a liquid crystal therebetween.
Electrostatic discharges of high voltage generated near the liquid crystal display panel of the liquid crystal display device during the manufacturing process or in the manufacturing environment may cause damage to signal lines such as a gate line and a data line within the liquid crystal display panel, to thin film transistors, and to other liquid crystal display panel components.
Accordingly, in order to protect a plurality of signal lines and a thin film transistor, or other components from the electrostatic discharge, an electrostatic discharge protection circuit is provided within the liquid crystal display panel.
FIG. 1 is a plan view showing a thin film transistor array substrate of a liquid crystal display panel that includes an electrostatic discharge protection circuit.
Referring to FIG. 1, a gate line 2 and a data line 4, a thin film transistor 6, and a pixel electrode 18 are located in a display area P1 of the thin film transistor array substrate. Herein, the gate line 2 and the data line 4 cross each other. The thin film transistor 6 is formed near a crossing of the gate line 2 and the data line 4. The pixel electrode 18 is connected to the thin film transistor 6.
A gate pad 3, a data pad 5, and an electrostatic discharge protection circuit 60 are located in a non-display area P2 of the thin film transistor array substrate. Herein, the gate pad 3 is connected to the gate line 2 to supply a gate voltage to the gate line. The data pad 5 is connected to the data line 4 to supply a data voltage to the data line 4. The electrostatic discharge protection circuit 60 is formed between the gate line 2 and a common line 57 and between the data line 4 and the common line 57. Herein, the common line 57 is connected to a common pad 55 to be supplied with a reference voltage from the common pad 55. Alternately, the electrostatic discharge protection circuit 60 may be connected to a ground electrode instead of the common line 57.
FIG. 2 is a schematic diagram showing details of the electrostatic discharge protection circuit 60 in FIG. 1.
Referring to FIG. 2, the electrostatic discharge protection circuit 60 is formed between the gate line 2 and the common line (or the ground electrode), or is formed between the common line (or the ground electrode) and the data line 4. FIG. 2 shows an example in which the electrostatic discharge protection circuit 60 is formed between the data line 4 and the common line 57.
The electrostatic discharge protection circuit 60 includes first to third transistors (T1 to T3). Herein, the first to third transistors (T1 to T3) are connected between the common line 57 and the data line 4.
If a voltage of more than a threshold voltage Vth for a transistor is applied to the common line 57, the first transistor T1 is turned-on to turn-on the third transistor T3 to allow a current from the common line 57 to be discharged toward the data line 4 via the third transistor T3. If a voltage of more than a threshold voltage Vth is applied to the data line 4, the second transistor T2 is turned-on to turn-on the third transistor T3 to allow a current from the data line 4 to be discharged toward the common line 57 via the third transistor T3. An electrostatic protection circuit having the identical structure and connected between the gate line 2 and the common line 57 may be driven using the same principle described above.
FIG. 3 is a diagram showing a I-V curve of transistors of the electrostatic discharge protection circuit in FIG. 2.
Referring to FIG. 3, a threshold voltage Vth of the first to third transistors T1 to T3 which are included in the electrostatic discharge protection circuit 60 of the related art is about 1V to 2V. On the other hand, a data voltage (or a liquid crystal driving voltage) is about 10V. Accordingly, if a voltage of about 2Vth is supplied to the first to third transistors T1 to T3, the first to third transistors T1 to T3 are turned-on.
For the a liquid crystal display panel of the related art, some current produced by the data voltage flows into the electrostatic discharge protection circuit 60 and is not available for driving the liquid crystal. Accordingly, the liquid crystal driving voltage is increased to compensate for the lost current. As a result of increasing the driving voltage, power consumption is increased when the liquid crystal display panel is driven to realize an image.